1. Field of the Invention
The present invention relates to an impedance matching device for use in an RF (radio frequency) amplifier such as a microwave amplifier, and in particular, to an electrically controllable impedance matching device (or tuner).
2. Description of the Related Art
In general, a microwave amplifier employs a mechanically controllable tuner or an electrically controllable tuner. FIG. 1 shows a known mechanically controllable tuner.
Referring to FIG. 1, the mechanically controllable tuner includes a body 10 being a transmission line, and a stub 20 coupled to the body 10. The mechanically controllable tuner is divided into a single stub tuner, a double stub tuner, and a triple stub tuner, according to the number of the stubs 20 coupled to the body 10. As is well known in the art a matching condition of the stub 20 depends on an interval between two lead wires of the stub 20, the position of the short-circuit wire, and a position where the transmission line is coupled to the stub. A length L2 of the stub 20 is adjustable by shorting (or short-circuiting) the lead wires using a mechanically controllable sliding rod 22, as shown by reference numeral 21. For an accurate control of the impedance, it is preferable to increase the number of the stubs 20 coupled to the body 10.
To adjust an impedance matching point of the tuner, the user should vary the length of the stub 20 manually, which will take much time. Besides, in this case, it is very difficult to set an accurate matching point. In addition, when the lead wires of the stub 20 are shorted by the sliding rod 22, the stub 20 may be damaged. Moreover, the stub 20 should increase in length, as the operating frequency band becomes lower.
FIG. 2 shows a schematic block diagram of a known electrically controllable tuner. Referring to FIG. 2, reference numeral 100 denotes an input terminal connected to a signal generator, reference numeral 110 denotes an input matching circuit 110 for controlling phase and amplitude of the signal input from the signal generator, reference numeral 120 denotes an amplifier for amplifying an output signal of the input matching circuit 110, and reference numeral 130 denotes an output matching circuit for controlling phase and amplitude of an output signal of the amplifier 120. Here, the amplifier 120 is composed of, by way of example, a transistor Q1 having a base connected to the output of the input matching circuit 110, an emitter connected to the ground, and a collector connected to an input of the output matching circuit 130. Alternatively, the amplifier 120 may be realized by an FET (Field Effect Transistor).
The known electrically controllable tuner controls the phase and amplitude of the signal by using the input and output matching circuits 110 and 130, to control input and output impedances seen respectively at input and output terminals 1 and 2 of the amplifier 120.
FIG. 3 shows a detailed circuit diagram of the input matching circuit 110. For reference, the output matching circuit 130 also has the same construction. Referring to FIG. 3, reference numeral 100 denotes the input terminal connected to the signal generator, and reference numeral 111 denotes a circulator. A termination 116 absorbs (or terminates) a reflected wave output from the circulator 111, if any. A variable attenuator 112 attenuates amplitude of the input signal received through the circulator 111 according to a DC attenuation control signal. A variable phase shifter 113 shifts the phase of the output signal of the variable attenuator 112 according to the DC phase control signal. The DC control signal is generated from a controller (not shown) which may be a microprocessor having a ROM (Read Only Memory) for storing a control program and a RAM (Random Access Memory) for temporarily storing data generated in the process of executing the control program. A directional coupler 114 splits an incident wave output of the variable phase shifter 113 from the reflected wave output of the amplifier 120. A power detector 115 measures an incident power and a reflection power of the incident wave and the reflected wave received from the directional coupler 114.
As can be appreciated from the foregoing, the known electrically controllable tuner is superior to the mechanically controllable tuner in that it can adjust the impedance matching point more accurately. However, it requires the expensive elements such as the circulator 111, the variable attenuator 112, the variable phase shifter 113, and the directional coupler 114. Further, the increase in number of the elements leads to an increase in size of the tuner undesirably. In addition, generally known electrically controllable phase shifters cannot shift the phase over 90.degree..